The present invention refers to an apparatus for determining the dew-point and/or the content of vapor in the air, particularly for the use in radio sondes. The apparatus comprises a dew-point mirror assembly, including a source of light, a reflector, a cooling element thermally coupled to the reflector, a temperature sensor for measuring the temperature of the reflector and a detecting sensor that receives the light emitted by the light source and reflected by the reflector. Included in the apparatus is a space or region that receives temperature sensitive components of the dew-point mirror assembly, whereby means are provided for stabilizing the temperature in this space or region.
In order to determine the content of vapor in the free atmosphere, radio sondes carried by a balloon are used as a matter of routine nowadays. Thereby, frequently the so-called adsorption moisture sensors are used that operate with the help of a hygroscopic plastic film. Reasonable results can be obtained with the aid of those sensors in a temperature range of between +50.degree. C. and -35.degree. C. Below a temperature of -35.degree. C., i.e. above approximately 7000 m height, depending on the weather and the latitude, the measurements are uncertain or not usable anymore because those sensors stop operating. The reason for that limit of possible measurements can be found in the physics of the adsorption measurement method.
For this reason, dew-point mirror assemblies are used more and more nowadays. The measuring method using those dew-point mirror assemblies is simple and well defined. A mirror is subjected to the air to be analyzed and cooled by means of a cooling element until it is misted up, i.e. until a condensate of water or, at lower temperatures, of ice is formed on its surface. Now, the temperature of the mirror is controlled in such a way that the condensate layer remains stable, i.e. the condensate layer does not evaporate and no new condensate is created. In this stable condition, the surface of the mirror has a temperature that corresponds to the vapor saturation temperature. The temperature of the surface of the mirror is measured by means of a thermometer integrated into the surface of the mirror. Based on the dew-point temperature and the simultaneously measured air temperature, the relative humidity of the air can be calculated.